Based on rapidly expanding needs for power generation and the desire to reduce the use of hydrocarbon fuels as well as reducing polluting emissions, fuel cells are expected to fill an important role in applications such as transportation and utility power generation. Fuel cells are highly efficient devices producing very low emissions, and having a potentially renewable fuel source and fast and convenient refueling. Fuel cells convert chemical energy to electrical energy through the oxidation of fuels such as hydrogen or methanol to form water and carbon dioxide. Hydrogen fuel, however, presents serious storage and transportation problems. For these reasons, significant attention has been paid to the development of liquid fuel based fuel cells, and more particularly, to fuel cells in which methanol is fed directly to the fuel cell without any pre-treatment, i.e., direct methanol fuel cells (DMFCs).
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction products. Fluid reactants are supplied to a pair of electrodes which are in contact with and separated by an electrolyte. The electrolyte may be a solid or a liquid, i.e., a supported liquid matrix. Solid electrolytes are comprised of solid ionomer or ion-exchange membrane disposed between two planar electrodes. The electrodes typically comprise an electrode substrate and an electrocatalyst layer disposed upon a major surface of the substrate. The electrode substrate typically comprises a sheet of porous, electrically conductive material, such as carbon cloth or carbon fiber paper. The electrode catalyst is typically in the form of finely comminuted metal, such as platinum, and is disposed on the surface of the electrode substrate in order to induce the desired electrochemical reaction. In a single cell, the electrodes are electronically coupled to provide a path for conducting electrons through an external load thereby producing electric current.
Direct methanol fuel cells are of particular interest over other fuel cell configurations due to a number of advantages. For example, because the methanol fuel is fed directly into the fuel cell a chemical pre-processing stage is unnecessary. In addition, bulky accessories for vaporization and humidification as in gas feed fuel cells are eliminated. Thus, direct methanol fuel cells are generally simple in construction and are suitable for many applications requiring portable power supplies.
In operation, the methanol fuel moves through the porous anode substrate and is oxidized at the anode electrocatalyst layer. At the cathode the oxidant, typically oxygen in air, moves through the porous cathode substrate and is reduced at the cathode electrocatalyst layer. In fuel cells of this type the reaction at the anode produces protons from the oxidation of methanol, as well as carbon dioxide. The anode and cathode reactions in direct methanol fuel cells are shown in the following equations: EQU Anode reaction: CH.sub.3 OH+H.sub.2 O.fwdarw.CO.sub.2 +6H.sup.+ +6e.sup.- EQU Cathode reaction: O.sub.2 +4H.sup.+ +4e.sup.-.fwdarw.2H.sub.2 O EQU Overall cell reaction: CH.sub.3 OH+1.5 O.sub.2.fwdarw.CO.sub.2 +2H.sub.2 O
The protons formed at the anode electrocatalyst migrate through the ion-exchange membrane from the anode to the cathode, while the electrons flow through an external load. At the cathode, the oxidant (oxygen) reacts with the protons to form water.
In these fuel cells, crossover of a reactant from one electrode to the other is undesirable. Reactant crossover may occur if the electrolyte is permeable to the reactant (methanol), i.e., some of the reactant introduced at a first electrode of the fuel cell may pass through the electrolyte to the second electrode, instead of reacting at the first electrode. Reactant crossover typically causes a decrease in both reactant utilization efficiency and fuel cell performance. Fuel cell performance is defined as the voltage output from the cell at a given current density or vice versa; the higher the voltage at a given current density or the higher the current density at a given voltage, the better the performance.
In solid polymer electrolyte direct methanol fuel cells the ion exchange membrane may be permeable to methanol, thus methanol which contacts the membrane prior to participating in the oxidation reaction can crossover to the cathode side. Diffusion of the methanol fuel from the anode to the cathode leads to a reduction in the fuel utilization efficiency and to fuel cell performance losses. Fuel efficiency utilization losses arise from methanol diffusion away from the anode because some of the methanol which would otherwise participate in the oxidation reaction at the anode and supply electrons to do work through the external circuit is lost. Methanol arriving at the cathode may be lost through vaporization into the oxidant stream, or may be oxidized at the cathode electrocatalyst, consuming oxidant (oxygen) in the process.
Methanol diffusion to the cathode also leads to a decrease in fuel cell performance. The oxidation of methanol at the cathode reduces the concentration of oxygen at the electrocatalyst and may effect access of the oxygen to the electrocatalyst because of mass transport issues. Furthermore, it has been well documented that for cathode electrocatalysts of the prior art, methanol oxidation poisons the catalytic activity of the electrocatalysts at the cathode. See, for example, Chu et al., J. Electrochem. Soc., Vol. 141, 1770-1773 (July 1994); Kuver et al., Electrochemica Acta, Vol. 43, 2527-2535 (1998); Cruickshank et al., J. Power Sources, Vol. 70, 40-47 (1998); and Kuver et al., J. Power Sources, Vol. 74, 211-218 (1998).
Several prior art patents have focused on reducing reactant crossover in electrochemical fuel cells, generally through modifications of the electrolyte membrane or the anode electrode itself. See, for example, U.S. Pat. Nos. 5,672,438; 5,672,439; 5,874,182; 5,849,428; 5,945,231; and 5,919,583. However, it has generally been found that electrolyte membranes which reduce methanol crossover also reduce fuel cell performance in that ion transfer is reduced. Essentially, a tradeoff is being made. Moreover, none of these prior art patents deal with improvements to the cathode electrocatalyst material itself in order to make the catalyst methanol tolerant.
When methanol crosses over from anode to cathode it causes two major detrimental effects on the cathode's function. First, of course, it decreases the efficiency of oxygen reduction at the cathode because the existing cathode catalysts facilitate methanol oxidation, consuming oxygen as well as leaving less reactive sites available for the oxygen reduction. Thus, a so-called "chemical short" occurs as methanol electro-oxidation occurs simultaneously with oxygen electro-reduction at the cathode. Essentially, as methanol crosses over it can be oxidized at the cathode according to the reaction: EQU CH.sub.3 OH+1.5 O.sub.2.fwdarw.CO.sub.2 +2H.sub.2 O
Secondly, this reaction is generally not complete, and typically results in the production of CO. The CO produced, in turn, poisons the catalytic activity of the existing catalysts which generally comprise platinum black. Thus, for state-of-the-art cathode electrocatalysts, methanol reactivity both decreases oxygen reduction and ultimately poisons the catalyst material itself.
The present invention provides novel electrocatalysts useful for oxygen reduction while at the same time being methanol "tolerant". Being "tolerant" to methanol means that these new catalysts do not oxidize methanol and, subsequently, are not poisoned by methanol or any of its oxidation products such as CO. Any methanol which crosses over to the cathode within the fuel cell is simply vented without reaction. Thus, no "chemical short" or poisoning of the catalyst occurs. Moreover, these new catalysts have excellent oxygen reduction catalytic activity.
The state-of-the-art electrocatalysts used for the reduction of oxygen generally comprise platinum or platinum-metal alloys on a substrate of carbon powder or the like. See, for example, U.S. Pat. Nos. 4,316,944; 4,822,699; 4,264,685; and 5,876,867. In addition, metal-containing macrocyclic compounds have been investigated for a number of years as fuel cell catalysts. These metal macrocyclic compounds include N.sub.4 -chelate compounds, such as phthalocyanines, porphyrins, and tetraazaannulenes. See, for example, U.S. Pat. No. 5,316,990 and Faubert et al., Electrochemica Acta, Vol. 43, pp.341-353, (1998). However, these catalysts have not proven to be methanol tolerant.
The present invention provides methanol tolerant electrocatalysts, and a method of making the same, fulfilling the needs of direct methanol fuel cells. These novel catalysts are excellent oxygen reduction materials while at the same time not causing methanol oxidation or being poisoned by the presence of methanol. The catalysts have double or multiple transition metal active sites and are produced through the heat-treatment of two or more transition metal-nitrogen chelates (macrocycles); for example, tetraphenylporphine iron (III) chloride and tetraphenylporphine cobalt (II) supported on conductive carbon or graphite nanostructures. It is believed that these nanostructured electrocatalysts have di-facial configurations wherein each of the two different transition metals interacts with the same oxygen molecule to catalyze reduction.